In a wide variety of practical applications there is a need for structures to vary the fluid-flow rate of flowing fluids without the production of noise and vibration. The term "throttling" is generally applied to the function of altering or adjusting fluid flow throughout a range of flow rates. The various structures by which the function is performed are generally called "throttling valves" to distinguish them from structures whose function is to open and close a flow path as a step function. To the extent that on-off valves are not opened and closed instantaneously, so that throttling noise and vibration may be produced therein at the time of opening or closure, the invention described herein is applicable to such valves as well, and they are included in the term "throttling valve".
A typical control valve for handling the flowing of high pressure fluids employs a structure in which the cross-sectional area of the flow path is altered. This type of structure generally produces substantial noise and vibration and is quite subject to damage from cavitation. However, the structures employed in this arrangement are, as a class, least expensive and most conveniently employed. Of particular interest herein is a structure for quieting of spool valves. In general, the noise, vibration and cavitation generated in orificial valves is an incident to the Venturi effect which attends movement of the fluid through the orificial opening. When the orifice has reduced cross-sectional area, or is throttled, fluid velocity is reduced, and its pressure energy is reduced. The energy difference results in turbulence following the orifice where it is transformed into increased internal temperature of fluid and into acoustic energy in the form of noise transmitted through the fluid and in vibration in the surrounding structure, some of which occurs at audible frequency. In extreme cases, the turbulence results in localized pressure reductions downstream from the orificial restriction sufficient to form vapor spaces or pockets. The vapor in these spaces is returned to liquid as the vapor bubble is imploded by the pressure of the medium surrounding the bubble. This phenomenon is called cavitation and results in noise and occasional erosion of adjacent surfaces of the valve structure. It will be appreciated that there are many applications for which it is desired to substantially reduce both the noise and the effects of cavitation in operation of spool valves. A similar useful effect is produced when the resulting noise is of a magnitude and frequency such that it is not readily transmitted to or through the surrounding structure.
There have been many structures devised in an attempt to deal with the noise, vibration and possible cavitation rsulting from operation of valves in high pressure systems. Most of these have involved some form of baffling means which operate in one way or another to divide the flow into finely divided streams. One such arrangement involves creating a baffle consisting of a number of successive layers of fine screen-like material which are held tightly together and preferably brazed since it is necessary to avoid mechanical vibration of the parts. Another type of structure which has been proposed and used to some extent includes baffles or sleeves of sintered metal. Both of these latter arrangements have proven unsuccessful for severe applications in that the amount of quieting provided is insufficient and that, in the case of the sintered elements, there is some inconsistency in structure which makes the results somewhat unpredictable. Another type of structure which has been used consists of a stack of disks having tortuous passageways etched on adjacent surfaces to thereby provide a large number of discrete flow paths with many turns as a means of frictionally inhibiting the flow across the stack. This arrangement can provide good quieting, but since it relies essentially on frictional losses, performance is quite susceptible to viscosity changes which are an inherent result of temperature changes.